Production of acetylene



A ril 30, 1968 A. BERGIER ET AL 3,381,051

PRODUCTION OF ACETYLENE Filed March 2, 1965 INVENTORS ANDREI BERGIER a JACQUES ROBERT BY mm M ATTORNEYS United States Patent 3,381,051 PRODUCTION OF ACETYLENE Andr Bergier, Pan, and Jacques Robert, Monein, France,

assignors, by mesne assignments, to Badische Anilin- & Soda-Fabrik AG., Ludwigshafen am Rhine, Germany, a corporation of Germmy Filed Mar. 2, 1965, Ser. No. 436,445 Claims priority, application France, Apr. 16, 1964, 971,092 Claims. (Cl. 260-679) ABSTRACT OF THE DISCLOSURE The present disclosure relates to preventing the precipitation of polymers, which normally deposit during the solvent extraction of acetylene from hydrocarbon gas pyrolysis in the apparatus through which the acetylene solution travels. This is accomplished by introducing into the solvent, alkali or alkaline earth hydroxides or low acid salts of alkali or alkaline earth cations in concentrations which may be less than 0.2%. This addition causes a lowering of the acidity and of the resistivity of the solvent. If these materials, such as sodium hydroxide, potassium hydroxide, lithium hydroxide, lime, magnesia, baryta, strontium hydroxide, or the low acid salts of these bases, such as the tribasic phosphates and borates, and particularly those of which the acids are volatile, such as the carbonates, are added to the solvent so that the resistivity of the solvent is lower than 300,000 ohm-cm, the deposits are stopped from forming. In such a case, the acidity of the solvent will simultaneously be below 0.002 equivalent per litre.

The present invention relates to improvements in the manufacture of acetylene from hydrocarbons; it concerns, more particularly, the solvent extraction of acetylene from the gas resulting from a pyrolysis, particularly from the partial combustion of hydrocarbons.

It is known to manufacture acetylene by hydrocarbon pyrolysis, particularly the partial combustion of different hydrocarbons, followed by an extraction of the acetylene with appropriate solvents. It is known that various solvents can be used for this purpose, such as for example kerosene, methanol, butyrolactone, acetonylacetone, dimethylformamide, N-methyl pyrrolidone, etc. This process encounters a quite serious difliculty which consists of polymers depositing on the effective surfaces of the various apparatus through which the solvent travels during the treatment.

These undesired deposits interfere with the circulation of the liquid by increasing the pressure drop in the apparatus; in addition, such deposits lower the heat transfer coefiicient. The exact mechanism of the formation of such deposits has not been fully explained at the present time; it is not even possible to say with certainty what the composition of the polymers is which are responsible for these deposits. Since the acetylene is generally accompanied by various other acetylenic compounds, for example, diacetylene and triacetylene, some olefines, carbon black, etc., it is frequently accepted that the polymers formed in the solution originate from the polymerisation of certain of these impurities, particularly the higher acetylenic impurities.

Various means have been proposed for preventing the formation of the polymers in question. Thus, the solvent has had added thereto different inhibitors of the polymerisation of diacetylene, particularly methylene blue, according to U.S. Patent 2,715,101; sodium nitrite, according to U.S. Patent 2,715,103; 2-vinyl pyridine, U.S. Patent 2,965,565; derivatives of aniline, such as amino phenol,

o-nitraniline, o-nitro-N-phenylamine, etc., indicated in U.S. Patent 2,907,801; pyridine in U.S. Patent 2,861,041; hexamethylene tetramine, U.S. Patents 2,715,104 and 2,915,138; hydrazine, U.S. Patent 2,915,139; p,p'-diphenyldiamine, U.S. Patent 2,715,105; alkanolamines, U.S. Patent 2,971,608; alkanolamine oxides, US. Patent 2,964,131; morpholines, U.S. Patent 2,907,803, etc.

Nevertheless, on the industrial level, these procedures and inhibitors have not provided a snfiicient improvement, and the problem of separating the solid deposits in the apparatus still remains. Contrary to the known processes, the present invention does not have for its object the inhibition of the actual formation of the polymers; it proposes to prevent the precipitation of these polymers or other deposits in the apparatus through which the acetylene solution travels. The utility thereof is thus very high, because even if inhibitors lower the content of polymers in the solvent, some polymers are still produced and can always be deposited in the apparatus.

The present invention provides a practical solution to the above problem. It avoids the deposition of polymers on the surfaces which come into contact with the solvent during the treatment of the gases containing the acetylene to be extracted. It thus becomes possible to permit an acetylene manufacturing installation to operate continuously for several months without any obstacle, due to any depositions of polymers. As regards the present state of the art, this constitutes a very important industrial advance.

The invention results from the surprising discovery that the deposition of polymers from the acetylene extracting solvent is avoided if the solvent has introduced thereinto alkali or alkaline earth hydroxides or low acid salts of alkali or alkaline earth cations in concentrations which may be less than 0.2%.

This addition causes a lowering of the acidity and of the resistivity of the solvent. In practice, the simplest embodiment consists in continuously or intermittently regulating the introduction of a solution (even highly diluted) or suspension of the hydroxide or of the basic salt in such manner as to keep the resistivity and the acidity of the solvent below a certain threshold. For example, it has been found that N-methyl pyrrolidone, as solvent for the extraction of acetylene, becomes somewhat acid during the operation and the acid content can reach 0.03 equivalent per litre. At the same time and in surprising manner, the resistivity can reach very high values of 300,000 ohm-cm. at 20 for a SO-cycle alternating current, and even 1,000,000 ohm-cm. under the same conditions. It is generally when this resistivity is high that there is a maximum formation of deposits of polymers in the circuits.

If any one of the products referred to above are added, namely, sodium hydroxide, potassium hydroxide, lithium hydroxide, lime, magnesia, baryta, strontium hydroxide, or the low acid salts of these bases, such as the tribasic phosphates and borates, and particularly those of which the acids are volatile, such as the carbonates, so that the resistivity of the solvent is lower than 300,000 ohm-cm. and preferably does not exceed 200,000 ohm-cm, the deposits are stopped from forming. It is preferred to have a resistivity lower than 70,000 ohm-cm. When the resistivity has been so reduced the acidity of the solvent will simultaneously be below 0.002 equivalent per litre and it is also thus possible to take advantage of the determination of this acidity for regulating the proportion of additive.

The process according to the invention thus consists in regulating the addition of basic substance to the solvent serving for the extraction of acetylene in such a manner that the acidity of the solvent never exceeds a a basic substance continuously or intermittently to the solvent, during the manufacture of the acetylene, in dry form if desired, but preferably in an aqueous solution. By way of non-limiting example, the following bases can, for example, be used: sodium hydroxide, potassium hydroxide, lithium hydroxide, lime, magnesia, baryta, trisodium or tripotassium phosphate, alkali borate, sodium or potassium carbonate, etc., or mixtures thereof. These bases may with advantage be added to the solvent in the form of aqueous solution, in which case the amount of water in the solvent should be maintained in the range 05-10%.

In order that the addition of a basic substance to the solvent may lower the acidity of the latter to below the desired limit, the basic substance should be chosen from those of which the ionisation constants are higher than 10 and preferably higher than 10 that is to say, those of which the pK of the combined acid, according to the Bronsted definition, when determined in water, is higher than 11 or better still higher than 12. In actual practice, organic bases such as pyridine, alkanolamines, arylamines, hexamethylene tetramines or other similar substances, of which the ionisation constants are generally below 10*, and range particularly between 10* and 10*, are not suitable in reasonable doses which are acceptable industrially, that is to say, below 0.5% by weight of the solvent and preferably below 0.1%

On the contrary, substances of basic character, of which the ionisation constant is higher than land more particularly of those of which the cation is alkali or alkaline earth, enable the object of the present invention to be achieved with very small doses which are below 0.1% by weight of the solvent and usually even with extremely small proportions of the order of 0.5 to 100 parts per million (p.p.m.).

While the alkali and alkaline earth bases and the purely basic salts of these bases, of which the anion is a weak acid, are perfectly suitable for bringing the acidity of the solvent to the desired value and thus preventing the precipitation of the polymer during manufacture, the insufiiciently basic salts, that is to say, those of which the pK of the acid is below 11, are not suitable, unless the acids which constitute them have a pK higher than and in addition are relatively highly volatile above a solution in the solvent which is used; this is the case of the carbonates and bicarbonates. Salts such as sodium nitrite, sodium acetate and the monobasic phosphates are not suitable, because in every case their pK is too low; salts such as the dibasic phosphates are not suitable because, their pK is too low and they are not volatile, while the salts such as trisodium phosphate are suitable, because they are sufiiciently basic.

An indication which is very useful in the control of the acidity of the solvent is the electrical resistivity of the latter. It is generally established that the solvent which causes the polymer to be deposited has a strong resistivity of the order of hundreds of thousands of ohm-cm., for example higher than 300,000 ohm-cm. or even more than 1 megohm-cm. at 20 C. with a 50-cy-cle alternating current. When the medium has been corrected according to the invention in order to cause the cessation of the polymer depositions, the resistivity is lower than 300,000 ohm-cm. and preferably does not exceed 200,000 ohmcm.; it is particularly advisable to have a resistivity lower than 70,000 ohm-cm.

It may be advantageous to add to the solvent, jointly with the basic substance, a small quantity of electrolyte, for example, 0.02 to 100 milliequivalents per litre, and preferably 0.1 to 10. Among the electrolytes capable of being used according to the invention, the following are mentioned as non-limiting examples: sulphates, chlorides, bromides, phosphates, formates, acetates, oxalates, oxybutyrates, etc., which are more or less soluble, of metals in general and particularly alkali or alkaline earth metals, and those of Groups I to IV and VIII of the Periodic System, especially salts of Na, K, Mg, Ca, Fe, Zn, Al, etc.

The compounds used according to the invention for avoiding polymer deposits are not restricted to the use with only N-methyl pyrrolidone or dimethylformamide as set out in the examples; they may be used in other organic solvents, such as a-cetonylacetone, acetone, triethylphosphate or others.

The manner in which the new process according to the invention is carried into effect is illustrated in the present specification by the comparison between the periods of manufacture of acetylene, the first under conventional conditions and the others with the application of a basic substance according to the invention. It is of course understood that this illustration is not limiting in any Way.

EXAMPLE 1 Acetylene is manufactured continuously by partial combustion of natural gas. The gas obtained has the following composition in percent by volume:

The gas thus obtained is treated in an installation shown diagrammatically in the accompanying drawing.

The gas 1 is first of all washed under atmospheric pressure in an absorber 2, in which it gives off to the N-methyl pyrrolidone, used as solvent, a part of the most soluble acetylene hydrocarbons thereof, of the naphthalene contained therein, and a good proportion of its humidity. The gas is then compressed to 11 kg./cm. without there being any formation of deposits, in the compressors 3,

while the solvent leaving the absorber 2, is sent 'to the main solvent circuit at the upper end of the column 10, to which reference will be made later.

Thus compressed, it is washed in counter-current with the N-methyl pyrrolidone containing a small percentage of water in column 4 which operates at ambient temperature. The solvent, having retained the acetylene and the heavy hydrocarbons, is expanded in the column '5, which receives tail gases at its base; a gas 7 to be recycled leaves the head of the column, while the purified acetylene leaves in the middle 8.

The solvent, which leaves the bottom 9 of this column 5, passes through line 26 into column 10 where it is degasified under heat, and then in vacuo, in a special column 1J1, from which leave the following: (1) at the top, gases which are reinjected into the preceding column 10; (2) in the middle, the higher acetylenic substances and water 13; (3) -in the base, the degasified solvent 14, the water content of which is brought to a very low value.

Water vapor and higher acetylenic hydrocarbons, which leave column 11 through line 13, pass into a column 22 at the top of which water is fed along line 23 for countercurrent washing the vapor and gas, in order to eliminate therefrom the fractions of entrained solvent. The vapor and gas thus freed from the solvent escape at 24 from column 22.. The aqueous liquid, which contains thethus eliminated solvent and which runs down from column 22 through conduit 25, is introduced into line 26 where it mixes with the solvent entering the degasifying column 10. Water fed at 23 constitutes the main adduction of water in the whole circuitry of the installation.

The vacuum column 11 serves specifically for separating the water fraction supplied by the treated gas or even injected voluntarily into the circuit; it constitutes a true distillation column, comprising at its base a boiler 15, where the main supply of heat in the circuit is produced.

It may be considered that in the distillation within column 111 reflux is constituted by the water fed through 23 into column 22, as this water returns to the top of 11, through lines 25 and 26 and through column 10. The major part of the solvent leaving the boiler returns, after cooling, by way of lines 14 and 16 into the first washing column 4 Operating in counter-current flow under pressure; a fraction 17 thereof is branched towards the preliminary washer 2 operating at atmospheric pressure, while a small portion 18, for example 2%, enters a dry evaporation installation 19 for the purpose of eliminating the polymer 21 formed during the manufacture; the solvent, thus freed from polymer, is recycled by way of a line 20.

The proportion of redistilled solvent is calculated so as to avoid the accumulation of polymer during the manufacture; in normal operation, the concentration of polymer in the solvent is 0.4% by weight.

Despite this precaution, after operating for several days, the presence of deposits was found on all the internal surfaces of the apparatus through which the solvent had travelled, It then became impossible to contihue manufacture without carrying out a cleaning of the heat exchangers approximately every 3 days. In addition, the continuous rise in the pressure drop in the columns, particularly in the column in which the higher acetylenes are separated out, became injurious to the operation from the third month and necessitated a stoppage for cleaning purposes after six months to permit the columns to be operated.

EXAMPLE 2 With the continuous production of acetylene, identical with Example 1, having found the same rapid formation of deposits, as in Example 1, there was initiated the addition to the circulating solvent of 0.01% by volume of an aqueous solution containing, in grams per litre, 7.0 g.p.l. of sodium carbonate and 11.0 g.p.l. of sodium bicarbonate.

In other words, into each cubic metre of solvent introduced continuously into the conduit assembly 16 and 17 was added, through line 27 for example, 100 ml. of the aforementioned aqueous solution.

It was then established that after operating for 4 days with this addition of electrolyte, the formation of polymer deposits in the apparatus had started to decrease, and stopped completely after one week. The manufacture continued normally for one month, with the aforementioned addition of base, without any deposition being produced in the apparatus.

In normal running, the solvent is neutral, whereas it previously contained an acidity of 0.022 equivalent per litre.

EXAMPLE 3 In a continuous production of acetylene, identical with Example 1, and having established after 3 weeks the same rapid formation of deposits as in Example 1, there was initiated the addition to the circulating solvent of 0.01% by volume of an aqueous solution containing, in grams per litre, the following electrolytes: 7.0 g.p.l. of sodium carbonate, 11.0 grid. of sodium bicarbonate, 5.5 g.p.l. of sodium chloride, and 4.0 g.p.l. of sodium sulphate.

It was then established that from the third day of operation with this addition of electrolyte, the formation of polymer deposits in the apparatus had decreased and ceased completely after one week. The manufacture continued normally for the three months which the test lasted, with the aforementioned addition of electrolyte, without any deposit being produced in the apparatus.

In normal running, electrical resistivities of 20,000 to 60,000 ohm-cm. were found for the circulating solvent, as compared with 500,000 to 1,500,000 in Example 1. On the other hand, the solvent remained neutral, whereas it had an acidity of 0.022 equivalent per litre during Example 1.

EXAMPLE 4 The manufacture was carried out as indicated in Example 3, and then the addition of the electrolyte solution was interrupted, thus reverting to the conditions of Example 1. It was then found that the acidity of the solvent increased in two weeks from 0.0015 eq./litre to 0.005 eq./litre and the resistivity from 50,000 to 325,000 ohm-cm. During this period, the increase of the pressure drop in the solvent-solvent exchangers, which had remained zero while the alkaline solution was added, started to increase in a progressively more rapid manner; thus, the total increase in the pressure drop in 15 days became 1 lag/cm. and it then became 0.2 kg./cm. during the fifteenth day. It could thus be seen that by continuing to work under these conditions, the exchangers would have to be cleaned in a very short time. After 15 days, in order to stop the soiling, the solvent in circulation had added thereto 0.0018% by volume of an aqueous solution containing 110 g./l. of sodium carbonate. As soon as this solution was introduced, the increases in the pressure drop started to move toward a zero value, which they maintained throughout the period during which the introduction of the carbonate solution was continued.

Correlatively, the acidity had fallen from 0.005 eq./ litre to 0.000 eq./litre and the resistivity from 325,000 ohm-cm. to 60,000 ohm-cm, this taking place in a period of 4 days.

EXAMPLES 5 TO 18 A solvent which is similar to that obtained at the end of Example 1 was used in a pilot unit at the rate of 10 litres per hour for washing 25 m. /h. of cracking gas in a one-stage washer, similar to the industrial initial washer 2 shown in the drawing.

The liquid leaving this initial washer had retained 50% of the diacetylene contained in the gas and more than of triacetylene, divinylacetylene and other heavy acetylenic substances. When it was maintained at C., polymers were produced in the solvent. On a laboratory scale, this solvent was treated continuously in a stripping installation arranged according to the same principle as the industrial set of columns 11 and 22, operating in the following manner: the solvent to be treated was introduced on the fifth plate from the top of the plate-type column (as 11) made of stainless steel; distilled water reached the top plate at the rate of 1.5 litres/hour; situated beneath the plate where the solvent is supplied were another 5 plates, and below that an evaporation boiler sim ilar to 15.

The vapours discharged at the top are composed of water and stripped organic products and a trace of N.M.P.; they are discharged by means of a high-capacity water ejector ensuring a pressure of mm. Hg in the installation, the pressure being kept constant by a regulator. Provision is made for the solvent withdrawn from the bottom of the column not to contain more than 2% of water, although in general the solvent may contain 0.5 to 10% water. This solvent is recycled to the top of the column, where the gas resulting from partial combustion of the methane is washed. The lower zone of the column serves to strip from the solvent the hydrocarbons which it contains, particularly the higher acetylenic hydrocarbons which have a strong tendency to polymerise. During this stripping operation, some of the monomers are polymerised and, in order to maintain a constant polymer concentration of 0.4% in the circuit, it is necessary to withdraw about one litre of solvent per hour and to replace it by the same amount of new solvent per hour. There is consequently reproduced here that which takes place on an industrial scale in the vacuum column 11 and the column 19.

Each test lasted 8 days without any interruption; the column was opened after this period, and, in the absence of additives, there were found to be abundant deposits of polymers on the caps of the plates and on the internal wall of the column; these deposits are such that, if the test is prolonged, they will completely obstruct the slots in the caps on the plates. Tests were carried out with various additives and they have given the results which are shown in the following table.

It is seen that very small proportions of basic substances according to the invention (tests to are sufficient to prevent the deposition of the polymers, while the substances known as inhibitors of the polymerisation of diacetylene (tests 11 to 17) do not produce this effect.

EXAMPLE 18 Tests similar to Examples 5, 8, 10, ll, 12 and 16 were carried out with dimethylformamide as solvent instead of N-methyl pyrrolidone; the same observations were made as regards the depositions of the polymers.

It will be observed to those skilled in the art that various changes may be made without departing from the spirit of the invention and therefore the invention is not limited to what is shown in the drawings and described in the specification, but only as indicated in the appended claims.

What is claimed is:

1. In a process for the recovery of acetylene from an acetylene-containing gas stream derived from hydrocarbons wherein said acetylene-containing gas stream is contacted with a solvent for the selective absorption of acetylene therefrom and the resultant solution is thereafter stripped to recover acetylene therefrom, said solution being contaminated by polymers formed during said process, the improvement comprising preventing precipitation of said polymers from said solution by utilizing with said solvent a strongly basic inorganic compound the cation of which is selected from the group consisting of alkali and alkaline earth cations, in such an amount that the resistivity of said solvent is maintained below 300,000 ohm-cm. at 20 C. for a 50-cycle alternating current and the acidity of said solvent is maintained below about 0.002 equivalent/litre during prolonged operation.

2. In a process for the recovery of acetylene from an acetylene-containing gas stream derived from hydrocarbons, wherein said acetylene-containing gas stream is contacted with N-methyl pyrrolidone for the selective absorption of acetylene therefrom and the resultant solution is thereafter stripped to recover acetylene therefrom, said solution being contaminated by polymers formed during said process, the improvement which comprises preventing precipitation of said polymers from said solution by adding the N-methyl pyrrolidone with such an amount of a strongly basic inorganic compound having its ionization constant higher than 10- that the acidity of the N-methyl pyrrolidone is below 0.002 equivalent per liter.

3. In a process for the recovery of acetylene from an acet ene-containing gas stream derived from hydrocarbons, wherein said acetylene-containing gas stream is contacted with N-methyl pyrrolidone for the selective absorption of acetylene therefrom and the resultant solution is thereafter stripped to recover acetylene therefrom, said solution being contaminated by polymers formed during said process, the improvement which comprises preventing precipitation of said polymers from said solution by adding the 1 -methyl pyrrolidone with 0.5 X 10- to 0.1% by weight of sodium carbonate in aqueous solution, thereby keeping the acidity of the N-methyl pyrrolidone at a value below 0.002 equivalent per liter.

4. A process in accordance with claim 1 wherein said strongly basic inorganic compound has a pK higher than ll when determined in water, said additive compound being employed in quantities of from 0.5 X 10' (0.5 ppm.) to 0.5% by weight of solvent.

5. A process in accordance with claim 4 wherein the quantity of said strongly basic inorganic compound is below 0.1% by weight of solvent.

6. A process in accordance with claim 1 wherein said solvent is selected from the group consisting of N-methyl pyrrolidone, dimethylformamide, acetone, acetonylacetone, triethylphosphate, kerosene, methanol, and butyrolactone.

7. A process in accordance with claim 1 wherein said strongly basic inorganic compound is utilized in an aqueous solution, the amount of water used being 05-10% of the solvent.

8. A process in accordance with claim 1 wherein said strongly basic inorganic compound is selected from the group consisting of sodium hydroxide, potassium hydroxide, lithium hydroxide, lime, magnesia, baryta, strontium hydroxide, trisodium phosphate, tripotassium phosphate, alkali borate, sodium carbonate, and potassium carbonate.

9. In a process for the recovery of acetylene from an acetylene containing gas stream derived from hydrocarbons, wherein said acetylene containing gas stream is contacted with N-methyl pyrrolidone for the selective absorption of acetylene therefrom and said solution is thereafter stripped to recover acetylene therefrom, said solution being contaminated by polymers formed during said process, the improvement which comprises preventing precipitation of said polymers from said solution by adding the N-methyl pyrrolidone employed with 0.5)(10 to 0.5 by weight thereof of a strongly basic inorganic compound having its ionisation constant higher than 10*.

10. In a process for the recovery of acetylene from an acetylene containing gas stream derived from hydrocarbons, wherein said acetylene containing gas stream is contacted with N-methyl pyrrolidone for the selective absorption of acetylene therefrom and said solution is thereafter stripped to recover acetylene therefrom, said solution being contaminated by polymers formed during said process, the improvement which comprises preventing precipitation of said polymers from said solution by adding the N-methyl pyrrolidone with 0.5x 10- to 0.1% by weight of sodium carbonate in aqueous solution, thereby keeping the acidity of the N-methyl pyrrolidone at a value below 0.002 equivalent per litre and the electric resistivity lower than 300,000 ohm-cm. at 20 C. for a -cycle alternating current.

References Cited UNITED STATES PATENTS 2,013,996 9/1935 Boumann 204-3l 2,599,649 6/1952 Lorenz 252-l FOREIGN PATENTS 925,469 5/ 1963 Great Britain.

DELBERT E. GANTZ, Primary Examiner.

.T. D. MYERS, Assistant Examiner. 

